There have been organic electroluminescent (organic EL) light-emitting devices that emit light by applying voltages to organic EL elements in which a light-emitting layer is interposed between a positive electrode (transparent electrode) and a negative electrode (back electrode). Since the organic EL light-emitting devices have advantages such as lightweight, small thickness, and low power consumption, they are used as backlights for liquid crystal displays and flat luminaires (Patent Document 1).
Although the organic EL light-emitting devices have the excellent features described above, they have following problems.
First, because an organic thin-film layer, such as a light-emitting layer, constituting an organic EL light-emitting device has a higher refractive index than air, total reflection of emitted light tends to occur at an interface. Therefore, there is a problem that light utilization efficiency is lower than 20% of the emitted light, i.e., light is mostly lost.
The organic EL light-emitting devices also have a problem of field-of-view dependence. Specifically, the light-emitting layer of an organic EL light-emitting device is made of a combination of a red-light-emitting layer, a green-light-emitting layer, and a blue-light-emitting layer. Because these light-emitting layers have different refractive indices, light is wavelength-separated at an interface between the light-emitting layers when the light-emitting surface of the organic EL light-emitting device is viewed obliquely. When the light is wavelength-separated, optical path lengths between the light-emitting layers change, and thus the light is seen with different hues depending on viewing angles. For example, when the organic EL light-emitting device is viewed from the front, such a change in optical path lengths is not likely to occur, and thus a change in color of light emitted from the organic EL light-emitting device is not likely to occur; however, when the device is viewed obliquely, light is seen with different hues due to a change in light path lengths between the light-emitting layers.
To solve the problems described above, a method has been proposed in which a diffusion layer is provided on the light-emission side of a light-emitting device to reduce variations in color tones by the light scattering effect of the diffusion layer (e.g., Patent Documents 2 and 3). Specifically, Patent Document 2 describes that a light-diffusing layer in which a light-diffusing material having a different refractive index from transparent resin is dispersed in the transparent resin is provided, and Patent Document 3 describes provision of a diffusion layer in which particles having a different refractive index from a transparent mother layer are scattered in the mother layer or a diffusion layer having a surface on which unevenness is provided by scattering relatively large particles in a transparent mother layer to scatter light.